Products made from polymeric materials have become a major environmental concern due to the difficulty in disposing of the spent products. The advantage of utilizing polymeric materials having infinite lifetimes to manufacture such items as disposable containers, fishing lines and nets, and disposable diapers has become an increasing environmental concern when these items are indiscriminately discarded into the environment. One solution proposed for reducing plastic wastes from these spent items is to design new polymers, or modify existing polymers, such that these newly developed plastics undergo degradation once the useful lifetime of the product is over and they are discarded.
Polymers may be made to degrade as they are continually used or after they are disposed by several different mechanisms. However, while each mechanism has its own advantages, each also has its faults.
Polymers may be made to degrade by photochemical means, for example. Thus, when the polymer is exposed to sunlight over protracted periods, it undergoes certain chemical changes resulting in its degradation. These polymers have a serious fault, however, in that the polymer will not degrade if the item is not exposed to the correct wavelength of sunlight. Thus, if the items made from a photodegradable polymer are discarded in a land fill, they will be buried, sunlight will not be able to reach the polymer, and degradation will not take place. Recycling of these materials is also difficult; there is no way of knowing how much degradation has occurred and the resulting new end groups on the polymer are ill defined.
Polymers may also be made to degrade by microbial means as well. Two such polymers are polycaprolactone (which is not a polymer of choice for many applications because of its poor overall physical characteristics) and blended polymers containing a enzymatic digestible component. One example of such a blended material is polyethylene which has mixed therewith an enzymatically degradable starch. When this polymer is discarded, enzymes produced by various bacterial and fungal species will attack the starch portions of the blended material, digesting it and leaving a very porous polyethylene residue which, unfortunately, stays in the environment.
Although aliphatic polyesters are well known to degrade by hydrolysis, aromatic polyesters that are commonly used for fibers and molded articles have been shown to undergo little, if any, degradation. A number of degradable polymers have received patents in the United States. For example, Stager and Minor have obtained U.S. Pat. No. 3,647,111 for a biodegradable soft drink can; Henry has obtained U.S. Pat. No. 3,676,401 for a photodegradable polyethylene film; Schmitt et al., have obtained U.S. Pat. No. 3,784,585 for water degradable resins containing blocks of polyglycolic acid units; Brackman has obtained U.S. Pat. No. 3,840,512 for degradable polyethylene; Guillet and Dan have obtained U.S. Pat. No. 3,878,169 for a photodegradable polyester; Coquard et al., have obtained U.S. Pat. No. 4,032,993 for implantable surgical articles which are bioresorbable and contain a copolyester of succinic acid and oxalic acid; and Yamamori et al., have obtained U.S. Pat. No. 4,482,701 for hydrolyzable polyester resins which contain therein a metallic salt of a hydroxy carboxylic acid.
In general, polyesters and copolyesters, as well as the preparation of these polymers, are described in both the scientific and patent literature. Carothers et al [Journal of the American Chemical Society 52:3292 (1930)], for example, describes the ester interchange reaction of various diols (such as ethylene glycol or 1.4-butanediol) and diesters to yield polymer. The preparation of polyesters of fiber-forming quality from dicarboxylic acids and diols is described in U.S. Pat. No. 2,952,652.